docs: def parse_sig(sig):
      """Parse signature integer values from s-expr."""
      label, sig = sig
      assert label == b'sig-val'
      algo_name = sig[0]
      parser = {b'rsa': _parse_rsa_sig,
                b'ecdsa': _parse_ecdsa_sig,
                b'eddsa': _parse_eddsa_sig,
                b'dsa': _parse_dsa_sig}[algo_name]
      return parser(args=sig[1:])